THE AMERICAN NATURALIST 



[Vol. XLV 



External agencies acting upon bacteria, crustaceans, 

 beetles, fungi, and some of the higher types of seed plants 

 have been seen to result in the appearance of new types 

 or genotypes, which have been found to transmit their 

 characters perfectly through so many generations as to 

 indicate practical permanency. 



In the greater majority of such cases of changes in 

 heredity, inclusive of Tower's cultures of beetles, Wol- 

 tereck's experiments with Daphnia, Morgan's results with 

 flies, and my own ovarial treatments of seed plants the 

 germ-plasm was exposed to the excitation of unusual cli- 

 matic factors, irradiation, concentrated nutritive media, 

 or of solutions of sugar or inorganic salts. 



The new qualities were seen to be fully displayed, and 

 to appear in a mutational manner in all of these in- 

 stances, although the new head form acquired by DapJii/ia 

 in Woltereck's experiments did not become fixed and 

 fully heritable until the organism had been kept under 

 the influence of the exciting agency for an extended period, 

 nearly two years. The most recent and one of the most 

 interesting series of results are those which show that 

 the influence of environic factors upon hybridizations 

 by excitation of the germ-plasm may alter materially 

 the results of the unions of identical pairs. This seems 

 to have been first suggested by De Vries and to have 

 been seen by MacDougal in hybrids of mutants of Oeno- 

 thera, while it has been established beyond doubt by the 

 extensive and conclusive results of Tower in crossing 

 beetles under various conditions that environic agencies 

 may exert a very marked effect upon the dominancy of 

 paired characters and the general composition of hybrid 

 progenies. A different phase of the matter is represented 

 by the experiments of Kanmieror, in which, characters 

 constituting temporary sexual dimorphism mendeli/e 

 when paired. Aberrants, sports or mutants have been 

 seen to arise and perpetuate themselves under unusual 

 culture conditions in yeasts and bacteria, their survival 

 being dependent upon pedigreed cultures in some cases; 

 and the successive generations were those resulting from 

 fissions, although in some cases spores were interposed. 



